U.S. Pat. No. 3,469,982 discloses a negative acting photopolymerizable film resist in which a photopolymerizable layer is sandwiched between a temporary support film and a temporary cover film. This film has gained widespread usage in the manufacture of printed circuits in which the cover film is removed, the photopolymerizable layer is laminated by heat and pressure to the surface, e.g., copper, of a substrate to be permanently modified, the layer is imagewise exposed to actinic radiation, the film support is removed, the unexposed areas of the layer are removed by solvent washout (development), and the resultant bared area of the copper surface is permanently modified, such as by etching or deposition of metal.
The photopolymerizable layer has to be nontacky so as to be handleable and not cold flow and yet has to have sufficient adhesion to the substrate surface and integrity (toughness) to survive solvent development.
Achievement of this adhesion is not instantaneous upon lamination but increases with time. A hold-up time of about thirty minutes or more and sometimes heating the layer may be required for sufficient adhesion that the support film can be removed without causing separation of the layer from the substrate surface. This holding time has the disadvantage of delaying the next processing step toward the manufacture of the printed circuit.
Attempts to decrease the hold-up time and to increase adhesion by changes in the composition of the photopolymerizable layer have detracted from other properties of the layer, e.g., reduced integrity during solvent development and/or reduced resistance to plating solutions. Changes in the compositions of the photopolymerizable layer to increase its ability to withstand solvent development tend to reduce adhesion of the layer to the substrate surface while increasing adhesion to the support film, making it difficult to remove the support film without detaching the photopolymerizable layer from the substrate surface.
Superimposed over these needs for faster and greater adhesion and greater integrity has been the desire for thinner photopolymerizable layers for economy, which increases the difficulty of satisfying these needs.